The present invention relates to a precision-controlled slit mechanism for use in an apparatus, such as an electron microscope, for analysis of an energy spectrum. More particularly, it relates to an adjustable slit for use in an energy-selected electron imaging filter to select part of an electron energy spectrum.
Modern electron microscopes are capable of imaging individual atoms in a thin sample. However, the images produced by the microscope alone contain no direct information concerning the chemical composition of the sample, and the image quality degrades significantly if the sample is more than a few atomic planes thick. The chemical information can be provided by selectively imaging only with electrons that have, while traversing the sample, experienced energy losses characteristic of particular atomic species. Thus, electrons that have experienced energy losses (other than the selected ones) are filtered out, and the energy spread of the electron beam used to form the image is considerably reduced. The image quality, normally degraded by the chromatic aberration of the imaging lenses, is thereby significantly improved.
The most widely applicable method of energy filtering is to illuminate the sample with a broad electron beam and produce an energy-selected image by an apparatus which forms a focused spectrum of electron energies, selects an energy pass-band, and transforms the spectrum back into an image. Such an apparatus is frequently called an energy-selected electron imaging filter. Attached to a high-performance transmission electron microscope, filters of this type can produce elemental concentration maps containing a large number of image points in a few seconds. They can also substantially improve the resolution of transmission electron images of thicker samples. This is especially important for biological samples which can normally be prepared only down to several hundred to several thousand atomic planes thick.
An example of an energy-selected imaging filter is described in commonly-owned U.S. Pat. No. 4,851,670, the disclosure of which is incorporated by reference. One aspect of such a filter is the need for a slit, through which the electrons pass, and the width of which can be controlled with precision. That is, the slit is used to select a part of an electron energy spectrum which is then transformed into an energy-selected image by later stages of the apparatus. Because the spectrum has a small dispersion, the control requirements for the slit are quite critical, namely that the width of the slit must be adjustable over the range of from less than 1 .mu.m to about 300 .mu.m with a reproducibility in achieving a given slit width of better than 0.5%. The slit must also be able to be completely withdrawn from the path of the electron beam or opened to several millimeters (for non-filtered operation) in width and then put back into the path of the beam and/or closed down to its original width.
Slits used in the prior art have either been of a fixed width or have used manual devices such as screw-type mechanisms for width adjustment. The former have the disadvantage of requiring replacement of one fixed-width slit for another every time a different portion in the energy spectrum is to be analyzed, while the latter have reproducibility problems because of the back lash and dead travel inherent in mechanical screw mechanisms. Further, prior adjustable width slits could not be interfaced with any automatic control apparatus as there was no ability to predict or measure the amount of back lash compensation needed every time the slit width was changed.
Accordingly, the need exists in this art for a precision selecting means defining an energy-loss spectrum interval whose width can be repeatedly and reproducibly set, to be used in an apparatus such as an energy-selected electron imaging filter.